Risk factors for early sexual maturation in Atlantic salmon in seawater farms in New Brunswick and Nova Scotia, Canada

Over the last 10 years, there have been sporadic increases in early sexual maturation (grilsing) in Atlantic salmon (AS) in seawater grow-out sites in New Brunswick with an apparent upward trend. Grilsing in AS reduces flesh quality and growth. With estimated gross revenue of $250 million in 2002, losses due to grilsing were estimated to be in the range of $11-$24 million, making grilsing one of the largest economic problems for the salmon aquaculture industry in New Brunswick at the present time. Reasons for grilsing have been experimentally investigated, but remain obscure with complex interactions of husbandry and environmental variables. Some factors believed to influence grilsing include feed intake, growth rate, size, lipid reserves, and water temperature. To maximize the value of farmed AS, knowledge of the factors that may trigger or predict high levels of grilsing would be beneficial to producers and fish health personnel. The objective of this study was to identify factors associated with an increase in grilsing at AS farms in New Brunswick and Nova Scotia, Canada. Results of a questionnaire given to company production managers were analyzed to determine the median prevalence of grilsing and to evaluate the impact of a variety of risk factors. The median prevalence of grilsing for the 266 study cages was 6.6% with a within-cage prevalence range of 0%-64.1%, and for the within farm prevalence for the 24 farms was 6.6% with a range of 1.6%-38.7%. Significant risk factors included average fish weight in the second August in seawater, the difference in temperature between the first February and the second September in seawater, size of cages, not feeding any moist feed, and feeding intensity. It is possible to adjust management factors to try to reduce the risk of grilsing. The presence of these risk factors may also be used as predictors for grilsing. If they are present, early harvesting of a cage may be warranted. Future studies should concentrate on collecting new information such as the use of lights at the cages to alter the physiologic process of grilsing. To facilitate this and other data collection for future studies, improved and potentially standardized records for companies are essential.